Selective plasma etching process for aluminum oxide patterning

ABSTRACT

This invention relates to a method for the selective and directed plasma etching of aluminum oxide, in which a mixture having the following constituents is used for etching: a. a polymerizing gas comprising at least partially unsaturated, perfluorinated hydrocarbon compounds; 
         b. optionally a compound having the formula CH x F y , where x=1-3 and y=4-x; c. oxygen; and d. a suitable carrier gas; and this mixture as a plasma, is brought into contact with the aluminum oxide to be etched.

The present invention relates to a method for the selective and directed plasma etching of aluminum oxide, and to the use of the method, in particular in semiconductor manufacturing.

Aluminum oxide has a high etching resistance toward etching plasmas used for etching silicon, silicon oxide, silicon oxynitride or silicon nitride. On account of this etching resistance, aluminum oxide is proposed as a hard mask or stop layer. However, an expedient use has failed hitherto primarily owing to the lack of a selective and anisotropic dry etching process for aluminum oxide.

There is likewise difficulty in classifying the dry etching of aluminum oxide in use as high-k dielectric (gate material in field-effect transistors or capacitory dielectric), as well as tunnel barrier (such as e.g. hard disk read heads), or in electroluminescent materials. In these cases, too, it would be desirable to have a method for etching aluminum oxide available.

On account of the poor selectivity of customary processes for anisotropic etching, thicker hard or resist mask layers are currently required for patterning the aluminum oxide and, moreover, the etching attack on the underlying material is relatively large due to the necessary overetch and the poor selectivity.

The sole selective etching process which is currently established is only a wet-chemical and thus isotropic process for removal of aluminum oxide. The disadvantages of this method includes an isotropic, i.e. undirected etching behavior, for which reason material removals and faults occur in particular at edge and boundary layer regions, which contributes to the poor controllability of the feature sizes. Wet-chemical processes are therefore suitable only to a greater extent for structures decreasing in size.

An anisotropic process for etching aluminum oxide with high selectivity has not yet been described heretofore.

Instead, high mask layer thicknesses are currently used in attempts to achieve as far as possible anisotropic patternings, and the removal of aluminum oxide often takes place by means of unadapted recipes, e.g. with Ar-based sputtering recipes.

Therefore, it is an object of the present invention to provide a method for the controlled etching of aluminum oxide.

This object is achieved by means of a method in accordance with Claim 1.

The present invention furthermore relates to the use of the etching method according to the invention for the selective etching of aluminum oxide with respect to silicon, photoresists and/or metals.

Furthermore, the present invention encompasses the use of the method according to the invention for etching barrier layers or tunnel layers made of aluminum oxide that are used e.g. in magnetic memories or in hard disk read heads.

The present invention also relates to the use of the method according to the invention in semiconductor manufacturing, in particular in the context of fabricating contact holes.

The present invention furthermore relates to a method for fabricating an aluminum oxide hard mask.

Claim 1 relates to a method for the selective and directed plasma etching of aluminum oxide, in which a mixture having the following constituents is used for etching:

-   -   a. a polymerizing gas comprising at least partially unsaturated,         perfluorinated hydrocarbon compounds;     -   b. optionally a compound having the formula CH_(x)F_(y), where         x=1, 2 or 3 and y=4-x;     -   c. oxygen; and     -   d. a suitable carrier gas;     -   and this mixture as a plasma, is brought into contact with the         aluminum oxide to be etched.

Although an optional constituent is specified under b., it is preferred for the volumetric proportion of b. to be greater than 0.

For the first time, a selective and anisotropic etching process for aluminum oxide has hereby been found, which at the same time is compatible with customary plasma etching chambers and can be used with utilization of customary gases, parameters and temperatures. This has been made possible by means of the adapted combination of the constituents, in particular by virtue of the simultaneous presence of polymerizing components and components effecting removal in sputtering/oxidizing fashion. It is assumed that the polymerization provides an at least temporary protection of surfaces against an excessively high degree of etching, while on the other hand removing constituents effect the etching and prevent an excessive formation of polymers. Constituent a. is a polymerizing constituent. Constituent b. presumably likewise contributes to polymerization, but due to the F component probably also effects a degree of removal. Constituent c. acts in oxidizing removing fashion and constituent d. acts principally as a dilution gas. It could not be expected that such a combination of constituents would enable a selective etching of aluminum oxide.

In a preferred embodiment of the present invention, C₄F₆ (1,1,2,3,4,4-hexafluoro-1,3-butadiene) and/or C₅F₈ is used as at least partially unsaturated, perfluorinated hydrocarbon compound. Noncyclic compounds are involved in this case. Particularly good selectivities with respect to silicon and resist materials have been observed with these compounds. C₄F₈ can likewise be used according to the invention.

According to the invention, aluminum oxide is understood to be Al₂O₃; however, the term also encompasses nonstoichiometric aluminum oxide as may occur in aluminum layer formations, if appropriate. Equally, the term silicon oxide is to be understood as silicon dioxide; nonstoichiometric ratios may be present in this case, too. The term silicon nitride encompasses various silicon nitrides, in particular Si₃N₄.

The compounds CH_(x)F_(y) are likewise predominantly contained in the gas mixture as a gas that supports the polymerization. In a preferred embodiment, CH₂F₂ is used as compound having the formula CH_(x)F_(y).

According to the invention, the carrier gas or dilution gas that is used may be any inert or largely inert gases, such as argon, xenon, helium and/or neon. The use of argon as carrier gas has turned out to be preferred, however. It is presumed that Ar is ionized in small proportions in the plasma and thus contributes to the removal of polymers forming on the surface.

The ratio of the constituents can be varied according to the invention. Preferably, the volumetric ratio of the constituents a:b:c:d is approximately 0.7-1.3:0-1:0.5-2:5-200, preferably approximately 0.8-1.2:0.4-0.8:0.6-1.4:10-100.

Although b. may be 0 in the first volumetric ratio specification, a value of approximately 0.1 is preferred as further lower limit.

A particularly preferred combination of constituents is the following composition:

-   -   a: C₄F₆; b:CH₂F₂; c:O₂; d:Ar. It is preferred, particularly in         the case of this composition, for the constituents a. to d. to         be present approximately in the following ratios:         a:b:c:d=1:0.6:0.8:20.

According to the invention, the process pressure may be varied by the person skilled in the art in accordance with the requirements. By lowering the pressure it is possible to improve the uniformity (at the same time with a reduced selectivity); conversely, higher pressure permits a higher selectivity with respect to resist with poorer uniformity of the etching. This may be compensated for by the person skilled in the art through adaptation of other process parameters (power, magnetic field strength, etc.).

According to the invention, it is preferred for the process pressure during the etching of aluminum oxide to be approximately 5 to 200 mtorr, more preferably approximately 15 to approximately 100 mtorr, even more preferably approximately 40 to approximately 80 mtorr.

The plasma power may be chosen and set by the person skilled in the art in accordance with the apparatus used and the etching requirements. When using an Applied Materials eMax 200 mm, (a magnetically enhanced reactive ion etch chamber), a power of approximately 1800 W at a process pressure of 40 mtorr and a temperature of −15° C. is a preferred value. The etching process may be carried out using a magnetic field or without a magnetic field. The magnetic field strength may be varied by the person skilled in the art. If a magnetic field is used, a value of approximately 100 gauss is a preferred guide value when using the above apparatus and at 1800 W and 40 mtorr.

Generally, preferred ranges of parameters within which the person skilled in the art may effect variation (relative to said type of installation and 200 mm wafers) are:

-   -   Power 500-2500 watts, pressure 5-200 mT, temperature −25 to 15°         C., magnetic field 0-120G, gas flow (total) 50-1000 sccm.

In the case of the composition that turned out to be particularly preferred above, where a:C₄F₆; b:CH₂F₂; c:O₂; d:Ar and where a:b:c:d=1:0.6:0.8:20, a selectivity of 4.6:1 with respect to Si and 3:1 for resist results at a process pressure of 40 mT (see examples).

The etching method according to the invention can thus be integrated well in semiconductor manufacturing methods and may be employed particularly where a selective etching with respect to silicon and resist is required. One important possibility for application of the method is in the formation of contact holes (contact hole etching), where it is possible to use aluminum oxide as a hard mask, an etching that is selective with respect to silicon, silicon nitride, silicon oxynitride or silicon oxide being carried out. Contact hole etching involves etching the aluminum oxide layer according to resist lithography in accordance with the method according to the invention, which is possible selectively with respect to Si or resist. The subsequent patterning of the underlying layer, such as e.g. silicon oxide or silicon nitride, is effected according to conventional methods using the patterned aluminum oxide layer as a hard mask. These etching methods attack the aluminum oxide layer only insignificantly or not at all, with the result that a good selectivity is ensured here as well.

After the etching, the etched aluminum oxide layer is preferably used as a hard mask for patterning an underlying layer, preferably made of silicon, silicon nitride or silicon oxide.

The method of the present invention is well suited to the controlled removal of aluminum oxide on Si, silicon oxynitride, silicon oxide and/or silicon nitride.

The method according to the invention may be used in particular for the directed, selective dry etching of aluminum oxide layers, preferably for the selective etching of aluminum oxide layers with respect to silicon and photoresist.

Aluminum oxide layers occur for example as tunnel layers or barrier layers in hard disk read heads or in magnetic memories. The method of the present invention may preferably be used for etching barrier layers or tunnel layers made of aluminum oxide that occur in magnetic memories or in hard disk read heads.

Generally the method according to the invention may preferably be used in semiconductor manufacturing in order to etch and/or pattern aluminum oxide layers in that context. Such a patterned layer may preferably be used as a hard mask for patterning underlying layers made of silicon, silicon nitride and/or silicon oxide, e.g. during contact hole etching.

Consequently, a further aspect of the present invention relates to a method for fabricating an aluminum oxide hard mask, having the steps of:

-   -   a. providing an aluminum oxide layer on a substrate, preferably         a silicon, silicon nitride, silicon oxynitride and/or silicon         oxide substrate;     -   b. providing a mask on the aluminum oxide layer;     -   c. etching the aluminum oxide layer by the method according to         one of Claims 1 to 8.

The use of highly polymerizing gases such as C₄F₆ or C₅F₈ in a mixture with Ar and CH_(x)F_(y) and O₂ enables, according to the invention, an aluminum oxide etching which is highly selective with respect to Si and resist. A factor that influences the etching is the selected ratio of the polymerizing gases (C₄F₆, C₅F₈, CH_(x)F_(y)) to oxygen and the corresponding dilution by Ar. Preferred ratios are specified above.

Advantages of the present invention are e.g.:

-   -   1. The etching process described facilitates the use of aluminum         oxide as a hard mask that has a good selectivity with respect to         Si, SiN and SiO₂.     -   2. Improved or controlled removal of aluminum oxide on Si, SiO         and SiN (high-K dielectrics e.g. as trench-dielectric or as gate         dielectric).     -   3. Etching of the tunnel barrier in magnetic memories (MRAM) is         made possible or improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of the selectivity of Al₂O₃ with respect to Si₃N₄, SiO₂, a resist and Si when using the etching method according to the invention.

FIGS. 2 to 7 diagrammatically show various stages in the production of contact holes using aluminum oxide as a hard mask, in order to elucidate the present invention by way of example.

EXAMPLES

1. Etching and Selectivity Tests

Unpatterned wafer slices to which an Al₂O₃ layer having a thickness of approximately 100 nm was applied by means of ALD (Atomic Layer Deposition with organometallic precursor) were provided with layers made of Si₃N₄, SiO₂, a resist (MUV (365 nm) resist from JSR; MUV=middle UV range) and Si at selected regions in the conventional manner. It may be assumed that other types of resist (e.g. for 248 nm/193 nm lithography) behave similarly.

Afterward, with each of the wafer slices coated in this way, a plasma etching was carried out with the following parameters: Pressure: 15, 40 and 100 mT Time:  60 s Power: 1800 W Magnetic field  100 G.

The apparatus used was the Applied Materials eMax 200 mm described above.

After etching, the surface alterations, i.e. etching rate and uniformity of the surface, were determined by ellipsometry. The uniformity is specified in percent as (maximum etching rate minus minimum etching rate)/(2×average etching rate).

The following results were obtained: Etching rate Pressure nm/min Uniformity/%  15 mT 74.1 ±20.9  40 mT 50.8 ±37.9 100 mT 22.6 ±56.4

The selectivities S were furthermore determined. S=etching rate Al₂O₃/etching rate reference material. The results of the selectivities of aluminum oxide with respect to various tested materials are illustrated graphically in FIG. 1. The illustration in each case shows mean values from two tests. At 40 mT, selectivities of approximately 4.6 and 3 were obtained with respect to Si and the resist. At 100 mT the values were >10:1.

2. Contact Hole Patterning

A following layer construction was produced according to conventional methods known to the person skilled in the art (from top to bottom):

-   -   Resist     -   Al₂O₃     -   Oxide     -   Si (or metal)

The following method steps were carried out:

Firstly, a contact hole lithography was effected in a conventional manner. Afterward, the Al₂O₃ was patterned by a process according to the invention, i.e. an etching method was carried out with a mixture comprising C₄F₆:CH₂F₂:O₂:Ar in the ratio 1:0.6:0.8:20 at a process pressure of 40 mT. Further parameters: Time:  60 s Power: 1800 W Magnetic field  100 G.

The apparatus used was the Applied Materials eMax 200 mm.

The resist was then removed (resist stripping) in a conventional manner and the oxide was then patterned using the Al₂O₃ as a hard mask. Stop on Si/metal. The Al₂O₃ may subsequently be removed wet-chemically, if required for process integration reasons.

In this way, the oxide lying below Al₂O₃ was able to be patterned and etched simply and effectively using Al₂O₃ as a hard mask. This example shows that the method according to the invention can generally be used for contact hole etching.

3. Deep Trench with Al₂O₃ Hard Mask (Storage Capacitor Patterning for DRAM)

A deep trench patterning is an etching with a very high aspect ratio into the crystalline Si. This etching may be effected, according to the invention, with very high selectivity with respect to the Al₂O₃ hard mask.

A following layer construction was produced according to conventional methods known to the person skilled in the art (from top to bottom):

-   -   Resist (˜150-350 nm)     -   Al₂O₃ (˜50-200 nm)     -   Si₃N₄ (pad nitride ˜100-200 nm)     -   SiO₂ (thin pad oxide)

The following method steps were carried out:

Firstly, a contact hole lithography was effected in a conventional manner. The Al₂O₃ was subsequently patterned by a process according to the invention, i.e. an etching method was carried out with a mixture comprising C₄F₆:CH₂F₂:O₂: Ar in the ratio 1:0.6:0.8:20 at a process pressure of 40 mT. Further parameters: Time:  60 s Power: 1800 W Magnetic field  100 G.

The apparatus used was the Applied Materials eMax 200 mm.

The resist was then removed (resist stripping) in a conventional manner and the silicon nitride was then patterned.

As an alternative, after contact hole lithography, the Al₂O₃ may be patterned by the above-described process according to the invention together with the Si₃N₄ patterning in one etching step. The resist stripping is then performed.

In accordance with this example, a relatively thick Si₃N₄ layer could be etched effectively using Al₂O₃ as a hard mask.

4. Contact Hole Etching

An exemplary embodiment of the present invention is illustrated diagrammatically in FIGS. 2 to 7 and explained in more detail below. A method for fabricating self-aligned contacts is involved in this case.

FIG. 2 shows an exemplary silicon semiconductor substrate 1 with a memory cell arrangement that is not illustrated in greater detail. 60 designates an active region, for example a common source/drain region of two memory cells. GS1, GS2 are two gate stacks lying next to one another, which are constructed from a polysilicon layer 10 with underlying (not illustrated) gate dielectric layer (e.g. gate oxide), if appropriate a silicide layer 20 and a silicon nitride cap 30 and also a sidewall oxide layer 40. CB designates the position at which a contact to the active region 60 is to be fabricated.

Between the two gate stacks GS1, GS2 it is necessary to provide a contact type CB, which makes electrical contact with the active region 60 between the two gate stacks GS1, GS2. Usually, the contact hole for the contact CB is etched separately from other contacts. In this case, the distance results, as is known, from the increasing miniaturization that leads to an increase in the number of chips per wafer and thus to a reduction of costs.

Afterward, as illustrated in FIG. 3, a silicon oxide layer, e.g. a BPSG layer (borophosphosilicate glass), designated by reference symbol 100, is deposited over the resulting structure. Said BPSG layer 100 is made to flow in a subsequent heat treatment, so that it does not leave any voids in particular between the closely adjacent gate stacks GS1, GS2.

In a subsequent method step (not illustrated), a planarizing ARC coating (anti-reflective coating) may be spun on, which compensates for the remaining unevennesses of the surface of the BPSG 100. If this does not suffice, a planarization, for example by means of chemical mechanical polishing (CMP), may also be effected after the heat treatment of the BPSG layer 100.

Afterward, as illustrated in FIG. 4, an Al₂O₃ layer, designated by reference symbol 110, is deposited on the resulting structure. This Al₂O₃ layer later serves as a hard mask for the selective etching of the underlying silicon oxide. Furthermore, as is illustrated in FIG. 4, a resist layer 120 for the later patterning of the aluminum oxide layer 110 is applied.

FIG. 5 shows the state after exposure of the resist in order to form a mask for the patterning of the Al₂O₃ layer.

FIG. 6 shows the state after carrying out the etching method according to the invention, e.g. with a mixture comprising C₄F₆:CH₂F₂:O₂:Ar in the ratio 1:0.6:0.8:20, at a process pressure of 40 mT. the method according to the invention is thus utilized for producing a hard mask made of aluminum oxide.

FIG. 7 then shows the state after selective etching of the contact hole and removal of the resist layer. The contact hole is subsequently filled. The aluminum oxide layer may be removed prior to the contact hole being filled, e.g. with tungsten, but may also remain and serve as a spacer from the substrate in order to keep capacitive couplings low.

The selection of the substrate material and the geometry are only by way of example and may be varied in many different ways. In particular, the present invention can be employed not only for the fabrication of contact holes but wherever aluminum oxide layers have to be etched selectively with respect to silicon, photoresists or metals or wherever silicon oxide, silicon nitride and/or silicon oxynitride have to be etched selectively with respect to aluminum oxide. 

1. Method for the selective and directed plasma etching of aluminum oxide, having the step of etching with a mixture having the following constituents: a. a polymerizing gas comprising at least partially unsaturated, perfluorinated hydrocarbon compounds; b. optionally a compound having the formula CH_(x)F_(y), where x=1, 2 or 3 and y=4-x; c. oxygen; and d. a suitable carrier gas; and this mixture as a plasma, is brought into contact with the aluminum oxide to be etched.
 2. Method according to claim 1, wherein C₄F₆ and/or C₅F₈ are used as the at least partially unsaturated, perfluorinated hydrocarbon compounds for gas a.
 3. Method according to claim 1, wherein CH₂F₂ is used as compound having the formula CH_(x)F_(y).
 4. Method according to claim 1, wherein argon is used as carrier gas.
 5. Method according to claim 1 wherein the volumetric ratio of the constituents a:b:c:d is approximately 0.7-1.3:0-1:0.5-2:5-200, preferably approximately 0.8-1.2:0.4-0.8:0.6-1.4:10-100.
 6. Method according to claim 1 wherein the following combination of constituents is used: a: C₄F₆; b: CH₂F₂; c: O₂; d: Ar.
 7. Method according to claim 1, wherein the constituents a. to d. are present in approximately the following ratios: a:b:c:d=1:0.6:0.8:20.
 8. Method according to claim 1, wherein the process pressure during the etching of aluminum oxide is approximately 15 to approximately 100 mtorr, preferably approximately 40 to approximately 80 mtorr.
 9. Method according to claim 1, wherein the method is effected as part of the fabrication of a semiconductor structure in order to produce an etched aluminum oxide layer.
 10. Method according to claim 9, wherein the etching for producing an etched aluminum oxide layer is followed by a patterning of an underlying layer, preferably made of silicon, silicon nitride or silicon oxide, the etched aluminum oxide layer being used as a mask.
 11. Use of the method according to claim 1, for controlled removal of aluminum oxide on Si, silicon oxide, silicon oxynitride or silicon nitride.
 12. Use of the method according to claim 1, for etching barrier layers or tunnel layers made of aluminum oxide that occur in magnetic memories or in hard disk read heads.
 13. Use of the method according to claim 1 in semiconductor manufacturing, in particular in the course of contact hole etching.
 14. Method for fabricating an aluminum oxide hard mask, having the steps of: a. providing an aluminum oxide layer on a substrate, preferably a silicon, silicon nitride, silicon oxynitride and/or silicon oxide substrate; b. providing a mask on the aluminum oxide layer; and c. etching the aluminum oxide layer by the method according to claim
 1. 